A crosslinking agent is an agent which mainly serves to chemically bond molecules of a polymeric compound to each other. The crosslinked structure formed with a crosslinking agent generally has improved properties, e.g., strength, heat resistance, and solvent resistance, as compared with an uncrosslinked polymeric compound.
Although various crosslinking agents have been proposed, compounds containing a hydrazide group have been proposed as crosslinking agents which are usable in an aqueous system without using any organic solvent, due to the recent increasing concern about environmental issues. For example, patent document 1 discloses a hydrazide-group-containing copolymer having a specific structural unit. Since compounds containing a hydrazide group are suitable for use in applications where a relatively high crosslinking rate is desired, patent document 2 discloses a feature wherein a compound containing a hydrazide group is used as an adhesive in the thermal recording layer and/or protective layer of a thermal recording medium.
A crosslinked polymer crosslinked with a crosslinking agent has a three-dimensional network structure formed therein. This crosslinked polymer is obtained by a crosslinking reaction between a linear polymer having reactive functional groups in the side chains or main chain thereof and a crosslinking agent.
As such linear polymer, various polymers are being investigated, such as acrylic resins, urethane-based resins, epoxy-based resins, and polyvinyl alcohol-based resins. Such linear polymers in which carbonyl groups have been introduced into the side chains or main chain are in extensive use because of the high reactivity thereof.
Of these, polyvinyl alcohol-based resins (hereinafter, polyvinyl alcohol is abbreviated to PVA), which are water-soluble resins, can be made to have water resistance by crosslinking, and are among the resins which are practically important from the standpoint of forming crosslinked polymers therefrom.
In the case of such PVA-based resins, it is preferable that a modified PVA-based resin into which carbonyl groups have been introduced should be used, from the standpoints of heightening the efficiency of crosslinking and forming a strong crosslinked structure. Examples of such modified PVA-based resins include an acetoacetyl group-containing PVA-based resin (hereinafter referred to as AA-modified PVA-based resin), a PVA-based resin containing a diacetone acrylamide structural unit (hereinafter referred to as DAAA-PVA-based resin), and a carboxylic-acid-modified PVA-based resin.
As stated above, compounds containing a hydrazide group are in extensive use as crosslinking agents for linear polymers, in particular, polymers having carbonyl groups, for example AA-modified PVA-based resins and DAAA-PVA-based resins. However, the crosslinked polymers obtained by crosslinking with compounds containing a hydrazide group have had a problem in that the crosslinked polymers take a color with the lapse of time depending on the storage environment. In order to overcome this problem, various investigations have been made so far.
Examples of such investigations include to incorporate a reducing agent as described in patent document 3.